Attempting to compare shotguns and rifles is similar to comparing lightning bugs and lightning. Not only do they operate at totally different pressures, but the pressure curve is different.
12 g- 11,500 PSI
308 Winchester- 62,000 psi
Perhaps your best measure would be to remove the barrel from a pump gun, such as the Mossberg 500 or Ithaca 37, and measure it. Very small bore shotguns, such as the .410 will have proportionally thicker chambers and barrels- not for internal pressures, but to withstand external abuses.
What we have here is... failure- to communicate.